Exploration 4 Chapter 3 Frame Relay

Frame Relay: An Efficient and Flexible WAN Technology Frame Relay has become the most widely used WAN technology in the world. Frame Relay has become the most widely used WAN technology in the world. Large enterprises, governments, ISPs, and small businesses use Frame Relay, primarily because of its price and flexibility. Large enterprises, governments, ISPs, and small businesses use Frame Relay, primarily because of its price and flexibility. Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Frame Relay reduces network costs by using less equipment, less complexity, and an easier implementation. Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines. Frame Relay provides greater bandwidth, reliability, and resiliency than private or leased lines. With increasing globalization and the growth of one-to-many branch office topologies, Frame Relay offers simpler network architecture and lower cost of ownership. With increasing globalization and the growth of one-to-many branch office topologies, Frame Relay offers simpler network architecture and lower cost of ownership.

The Frame Relay WAN Frame Relay handles volume and speed efficiently by combining the necessary functions of the data link and network layers into one simple protocol. Frame Relay handles volume and speed efficiently by combining the necessary functions of the data link and network layers into one simple protocol. As a data link protocol, Frame Relay provides access to a network, delimits and delivers frames in proper order, and recognizes transmission errors through a standard Cyclic Redundancy Check. As a data link protocol, Frame Relay provides access to a network, delimits and delivers frames in proper order, and recognizes transmission errors through a standard Cyclic Redundancy Check. As a network protocol, Frame Relay provides multiple logical connections over a single physical circuit and allows the network to route data over those connections to its intended destinations. As a network protocol, Frame Relay provides multiple logical connections over a single physical circuit and allows the network to route data over those connections to its intended destinations. Frame Relay operates between an end-user device, such as a LAN bridge or router, and a network. Frame Relay operates between an end-user device, such as a LAN bridge or router, and a network. The network itself can use any transmission method that is compatible with the speed and efficiency that Frame Relay applications require. The network itself can use any transmission method that is compatible with the speed and efficiency that Frame Relay applications require. Some networks use Frame Relay itself, but others use digital circuit switching or ATM cell relay systems. Some networks use Frame Relay itself, but others use digital circuit switching or ATM cell relay systems.

Frame Relay Operation The connection between a DTE device and a DCE device consists of both a physical layer component and a link layer component: The connection between a DTE device and a DCE device consists of both a physical layer component and a link layer component: The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the most commonly used physical layer interface specifications is the RS-232 specification. The physical component defines the mechanical, electrical, functional, and procedural specifications for the connection between the devices. One of the most commonly used physical layer interface specifications is the RS-232 specification. The link layer component defines the protocol that establishes the connection between the DTE device, such as a router, and the DCE device, such as a switch. The link layer component defines the protocol that establishes the connection between the DTE device, such as a router, and the DCE device, such as a switch.

Frame Relay Operation cont. When carriers use Frame Relay to interconnect LANs, a router on each LAN is the DTE. When carriers use Frame Relay to interconnect LANs, a router on each LAN is the DTE. A serial connection, such as a T1/E1 leased line, connects the router to the Frame Relay switch of the carrier at the nearest point-of-presence (POP) for the carrier. A serial connection, such as a T1/E1 leased line, connects the router to the Frame Relay switch of the carrier at the nearest point-of-presence (POP) for the carrier. The Frame Relay switch is a DCE device. Network switches move frames from one DTE across the network and deliver frames to other DTEs by way of DCEs. The Frame Relay switch is a DCE device. Network switches move frames from one DTE across the network and deliver frames to other DTEs by way of DCEs. Computing equipment that is not on a LAN may also send data across a Frame Relay network. The computing equipment uses a Frame Relay access device (FRAD) as the DTE. Computing equipment that is not on a LAN may also send data across a Frame Relay network. The computing equipment uses a Frame Relay access device (FRAD) as the DTE. The FRAD is sometimes referred to as a Frame Relay assembler/dissembler and is a dedicated appliance or a router configured to support Frame Relay. The FRAD is sometimes referred to as a Frame Relay assembler/dissembler and is a dedicated appliance or a router configured to support Frame Relay. It is located on the customer's premises and connects to a switch port on the service provider's network. In turn, the service provider interconnects the Frame Relay switches. It is located on the customer's premises and connects to a switch port on the service provider's network. In turn, the service provider interconnects the Frame Relay switches.

Virtual Circuits The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection through a Frame Relay network between two DTEs is called a virtual circuit (VC). The circuits are virtual because there is no direct electrical connection from end to end. The connection is logical, and data moves from end to end, without a direct electrical circuit. The connection is logical, and data moves from end to end, without a direct electrical circuit. With VCs, Frame Relay shares the bandwidth among multiple users and any single site can communicate with any other single site without using multiple dedicated physical lines. With VCs, Frame Relay shares the bandwidth among multiple users and any single site can communicate with any other single site without using multiple dedicated physical lines. There are two ways to establish VCs: There are two ways to establish VCs: SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION). SVCs, switched virtual circuits, are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION). PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs. PVCs, permanent virtual circuits, are preconfigured by the carrier, and after they are set up, only operate in DATA TRANSFER and IDLE modes. Note that some publications refer to PVCs as private VCs.

Multiple VCs Frame Relay is statistically multiplexed, meaning that it transmits only one frame at a time, but that many logical connections can co-exist on a single physical line. Frame Relay is statistically multiplexed, meaning that it transmits only one frame at a time, but that many logical connections can co-exist on a single physical line. The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints. The Frame Relay Access Device (FRAD) or router connected to the Frame Relay network may have multiple VCs connecting it to various endpoints. Multiple VCs on a single physical line are distinguished because each VC has its own DLCI. Multiple VCs on a single physical line are distinguished because each VC has its own DLCI. Remember that the DLCI has only local significance and may be different at each end of a VC. Remember that the DLCI has only local significance and may be different at each end of a VC. This capability often reduces the equipment and network complexity required to connect multiple devices, making it a very cost-effective replacement for a mesh of access lines. This capability often reduces the equipment and network complexity required to connect multiple devices, making it a very cost-effective replacement for a mesh of access lines. With this configuration, each endpoint needs only a single access line and interface. With this configuration, each endpoint needs only a single access line and interface. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the VCs, rather than on the maximum bandwidth requirement. More savings arise as the capacity of the access line is based on the average bandwidth requirement of the VCs, rather than on the maximum bandwidth requirement.

The Frame Relay Encapsulation Process Frame Relay takes data packets from a network layer protocol, such as IP or IPX, encapsulates them as the data portion of a Frame Relay frame, and then passes the frame to the physical layer for delivery on the wire. Frame Relay takes data packets from a network layer protocol, such as IP or IPX, encapsulates them as the data portion of a Frame Relay frame, and then passes the frame to the physical layer for delivery on the wire.

DLCI The 10-bit DLCI is the essence of the Frame Relay header. The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel is represented by a unique DLCI. Each virtual connection that is multiplexed onto the physical channel is represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.

FCS The FCS determines whether any errors in the Layer 2 address field occurred during transmission. The FCS determines whether any errors in the Layer 2 address field occurred during transmission. The FCS is calculated prior to transmission by the sending node, and the result is inserted in the FCS field. The FCS is calculated prior to transmission by the sending node, and the result is inserted in the FCS field. At the distant end, a second FCS value is calculated and compared to the FCS in the frame. At the distant end, a second FCS value is calculated and compared to the FCS in the frame. If the results are the same, the frame is processed. If the results are the same, the frame is processed. If there is a difference, the frame is discarded. If there is a difference, the frame is discarded. Frame Relay does not notify the source when a frame is discarded. Frame Relay does not notify the source when a frame is discarded. Error control is left to the upper layers of the OSI model. Error control is left to the upper layers of the OSI model.

Star Topology (Hub and Spoke) The simplest WAN topology is a star. The simplest WAN topology is a star. Connections to each of the remote sites act as spokes. Connections to each of the remote sites act as spokes. In a star topology, the location of the hub is usually chosen by the lowest leased-line cost. In a star topology, the location of the hub is usually chosen by the lowest leased-line cost. When implementing a star topology with Frame Relay, each remote site has an access link to the Frame Relay cloud with a single VC. When implementing a star topology with Frame Relay, each remote site has an access link to the Frame Relay cloud with a single VC.

Full Mesh Topology A full mesh topology suits a situation in which the services to be accessed are geographically dispersed and highly reliable access to them is required. A full mesh topology suits a situation in which the services to be accessed are geographically dispersed and highly reliable access to them is required. A full mesh topology connects every site to every other site. Using leased-line interconnections, additional serial interfaces and lines add costs. A full mesh topology connects every site to every other site. Using leased-line interconnections, additional serial interfaces and lines add costs. Using Frame Relay, a network designer can build multiple connections simply by configuring additional VCs on each existing link. Using Frame Relay, a network designer can build multiple connections simply by configuring additional VCs on each existing link. This software upgrade grows the star topology to a full mesh topology without the expense of additional hardware or dedicated lines. This software upgrade grows the star topology to a full mesh topology without the expense of additional hardware or dedicated lines. Since VCs use statistical multiplexing, multiple VCs on an access link generally make better use of Frame Relay than single VCs. Since VCs use statistical multiplexing, multiple VCs on an access link generally make better use of Frame Relay than single VCs. Service providers will charge for the additional bandwidth, but this solution is usually more cost effective than using dedicated lines. Service providers will charge for the additional bandwidth, but this solution is usually more cost effective than using dedicated lines.

Partial Mesh Topology For large networks, a full mesh topology is seldom affordable because the number of links required increases dramatically. For large networks, a full mesh topology is seldom affordable because the number of links required increases dramatically. The issue is not with the cost of the hardware, but because there is a theoretical limit of less than 1,000 VCs per link. In practice, the limit is less than that. The issue is not with the cost of the hardware, but because there is a theoretical limit of less than 1,000 VCs per link. In practice, the limit is less than that. For this reason, larger networks are generally configured in a partial mesh topology. For this reason, larger networks are generally configured in a partial mesh topology. With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh. The actual pattern is dependant on the data flow requirements. With partial mesh, there are more interconnections than required for a star arrangement, but not as many as for a full mesh. The actual pattern is dependant on the data flow requirements.

Inverse ARP Before a Cisco router is able to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination. Before a Cisco router is able to transmit data over Frame Relay, it needs to know which local DLCI maps to the Layer 3 address of the remote destination. Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk. Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk. This address-to-DLCI mapping can be accomplished either by static or dynamic mapping. This address-to-DLCI mapping can be accomplished either by static or dynamic mapping. Inverse ARP Inverse ARP The Inverse Address Resolution Protocol (ARP) obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks. The Inverse Address Resolution Protocol (ARP) obtains Layer 3 addresses of other stations from Layer 2 addresses, such as the DLCI in Frame Relay networks. It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of VCs are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these VCs can be used. It is primarily used in Frame Relay and ATM networks, where Layer 2 addresses of VCs are sometimes obtained from Layer 2 signaling, and the corresponding Layer 3 addresses must be available before these VCs can be used. Whereas ARP translates Layer 3 addresses to Layer 2 addresses, Inverse ARP does the opposite. Whereas ARP translates Layer 3 addresses to Layer 2 addresses, Inverse ARP does the opposite.

Dynamic address mapping Dynamic address mapping relies on Inverse ARP to resolve a next hop network protocol address to a local DLCI value. Dynamic address mapping relies on Inverse ARP to resolve a next hop network protocol address to a local DLCI value. The Frame Relay router sends out Inverse ARP requests on its PVC to discover the protocol address of the remote device connected to the Frame Relay network. The Frame Relay router sends out Inverse ARP requests on its PVC to discover the protocol address of the remote device connected to the Frame Relay network. The router uses the responses to populate an address-to-DLCI mapping table on the Frame Relay router or access server. The router uses the responses to populate an address-to-DLCI mapping table on the Frame Relay router or access server. The router builds and maintains this mapping table, which contains all resolved Inverse ARP requests, including both dynamic and static mapping entries. The router builds and maintains this mapping table, which contains all resolved Inverse ARP requests, including both dynamic and static mapping entries. On Cisco routers, Inverse ARP is enabled by default for all protocols enabled on the physical interface. Inverse ARP packets are not sent out for protocols that are not enabled on the interface. On Cisco routers, Inverse ARP is enabled by default for all protocols enabled on the physical interface. Inverse ARP packets are not sent out for protocols that are not enabled on the interface.

static address mapping The user can choose to override dynamic Inverse ARP mapping by supplying a manual static mapping for the next hop protocol address to a local DLCI. The user can choose to override dynamic Inverse ARP mapping by supplying a manual static mapping for the next hop protocol address to a local DLCI. A static map works similarly to dynamic Inverse ARP by associating a specified next hop protocol address to a local Frame Relay DLCI. A static map works similarly to dynamic Inverse ARP by associating a specified next hop protocol address to a local Frame Relay DLCI. You cannot use Inverse ARP and a map statement for the same DLCI and protocol. You cannot use Inverse ARP and a map statement for the same DLCI and protocol. An example of using static address mapping is a situation in which the router at the other side of the Frame Relay network does not support dynamic Inverse ARP for a specific network protocol. An example of using static address mapping is a situation in which the router at the other side of the Frame Relay network does not support dynamic Inverse ARP for a specific network protocol. To provide accessibility, a static mapping is required to complete the remote network layer address to local DLCI resolution. To provide accessibility, a static mapping is required to complete the remote network layer address to local DLCI resolution.

Local Management Interface (LMI) Basically, the LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE). Basically, the LMI is a keepalive mechanism that provides status information about Frame Relay connections between the router (DTE) and the Frame Relay switch (DCE). Every 10 seconds or so, the end device polls the network, either requesting a dumb sequenced response or channel status information. Every 10 seconds or so, the end device polls the network, either requesting a dumb sequenced response or channel status information. If the network does not respond with the requested information, the user device may consider the connection to be down. If the network does not respond with the requested information, the user device may consider the connection to be down. When the network responds with a FULL STATUS response, it includes status information about DLCIs that are allocated to that line. When the network responds with a FULL STATUS response, it includes status information about DLCIs that are allocated to that line. The end device can use this information to determine whether the logical connections are able to pass data. The end device can use this information to determine whether the logical connections are able to pass data.

LMI Extensions VC status messages - Provide information about PVC integrity by communicating and synchronizing between devices, periodically reporting the existence of new PVCs and the deletion of already existing PVCs. VC status messages prevent data from being sent into black holes (PVCs that no longer exist). VC status messages - Provide information about PVC integrity by communicating and synchronizing between devices, periodically reporting the existence of new PVCs and the deletion of already existing PVCs. VC status messages prevent data from being sent into black holes (PVCs that no longer exist). Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients. Multicasting supports the efficient delivery of routing protocol messages and address resolution procedures that are typically sent to many destinations simultaneously. Multicasting - Allows a sender to transmit a single frame that is delivered to multiple recipients. Multicasting supports the efficient delivery of routing protocol messages and address resolution procedures that are typically sent to many destinations simultaneously. Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Global addressing makes the Frame Relay network resemble a LAN in terms of addressing, and ARPs perform exactly as they do over a LAN. Global addressing - Gives connection identifiers global rather than local significance, allowing them to be used to identify a specific interface to the Frame Relay network. Global addressing makes the Frame Relay network resemble a LAN in terms of addressing, and ARPs perform exactly as they do over a LAN. Simple flow control - Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface. It is intended for those devices whose higher layers cannot use the congestion notification bits and need some level of flow control. Simple flow control - Provides for an XON/XOFF flow control mechanism that applies to the entire Frame Relay interface. It is intended for those devices whose higher layers cannot use the congestion notification bits and need some level of flow control.

Frame Relay Configuration Tasks

Enable Frame Relay Encapsulation Step 1. Setting the IP Address on the Interface Step 1. Setting the IP Address on the Interface Step 2. Configuring Encapsulation Step 2. Configuring Encapsulation Step 3. Setting the Bandwidth Step 3. Setting the Bandwidth Step 4. Setting the LMI Type (optional) Step 4. Setting the LMI Type (optional)

Configuring a Static Frame Relay Map Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk, and the address-to-DLCI mapping can be accomplished either by dynamic or static address mapping. Cisco routers support all network layer protocols over Frame Relay, such as IP, IPX, and AppleTalk, and the address-to-DLCI mapping can be accomplished either by dynamic or static address mapping. Dynamic mapping is performed by the Inverse ARP feature. Because Inverse ARP is enabled by default, no additional command is required to configure dynamic mapping on an interface. Dynamic mapping is performed by the Inverse ARP feature. Because Inverse ARP is enabled by default, no additional command is required to configure dynamic mapping on an interface. Static mapping is manually configured on a router. Establishing static mapping depends on your network needs. To map between a next hop protocol address and a DLCI destination address, use the frame-relay map protocol protocol-address dlci [broadcast] command. Static mapping is manually configured on a router. Establishing static mapping depends on your network needs. To map between a next hop protocol address and a DLCI destination address, use the frame-relay map protocol protocol-address dlci [broadcast] command.

Using the Broadcast Keyword Frame Relay, ATM, and X.25 are non-broadcast multiple access (NBMA) networks. Frame Relay, ATM, and X.25 are non-broadcast multiple access (NBMA) networks. NBMA networks allow only data transfer from one computer to another over a VC or across a switching device. NBMA networks allow only data transfer from one computer to another over a VC or across a switching device. NBMA networks do not support multicast or broadcast traffic, so a single packet cannot reach all destinations. This requires you to broadcast to replicate the packets manually to all destinations. NBMA networks do not support multicast or broadcast traffic, so a single packet cannot reach all destinations. This requires you to broadcast to replicate the packets manually to all destinations. Some routing protocols may require additional configuration options. For example, RIP, EIGRP and OSPF require additional configurations to be supported on NBMA networks. Some routing protocols may require additional configuration options. For example, RIP, EIGRP and OSPF require additional configurations to be supported on NBMA networks. Because NBMA does not support broadcast traffic, using the broadcast keyword is a simplified way to forward routing updates. Because NBMA does not support broadcast traffic, using the broadcast keyword is a simplified way to forward routing updates. The broadcast keyword allows broadcasts and multicasts over the PVC and, in effect, turns the broadcast into a unicast so that the other node gets the routing updates. The broadcast keyword allows broadcasts and multicasts over the PVC and, in effect, turns the broadcast into a unicast so that the other node gets the routing updates.

Split Horizon By default, a Frame Relay network provides NBMA connectivity between remote sites. By default, a Frame Relay network provides NBMA connectivity between remote sites. NBMA clouds usually use a hub-and-spoke topology. NBMA clouds usually use a hub-and-spoke topology. Unfortunately, a basic routing operation based on the split horizon principle can cause reachability issues on a Frame Relay NBMA network. Unfortunately, a basic routing operation based on the split horizon principle can cause reachability issues on a Frame Relay NBMA network. Recall that split horizon is a technique used to prevent a routing loop in networks using distance vector routing protocols. Recall that split horizon is a technique used to prevent a routing loop in networks using distance vector routing protocols. Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface. Split horizon updates reduce routing loops by preventing a routing update received on one interface to be forwarded out the same interface.

Frame Relay Subinterfaces Frame Relay can partition a physical interface into multiple virtual interfaces called subinterfaces. Frame Relay can partition a physical interface into multiple virtual interfaces called subinterfaces. A subinterface is simply a logical interface that is directly associated with a physical interface. A subinterface is simply a logical interface that is directly associated with a physical interface. Therefore, a Frame Relay subinterface can be configured for each of the PVCs coming into a physical serial interface. Therefore, a Frame Relay subinterface can be configured for each of the PVCs coming into a physical serial interface. To enable the forwarding of broadcast routing updates in a Frame Relay network, you can configure the router with logically assigned subinterfaces. To enable the forwarding of broadcast routing updates in a Frame Relay network, you can configure the router with logically assigned subinterfaces. A partially meshed network can be divided into a number of smaller, fully meshed, point-to-point networks. A partially meshed network can be divided into a number of smaller, fully meshed, point-to-point networks. Each point-to-point subnetwork can be assigned a unique network address, which allows packets received on a physical interface to be sent out the same physical interface because the packets are forwarded on VCs in different subinterfaces. Each point-to-point subnetwork can be assigned a unique network address, which allows packets received on a physical interface to be sent out the same physical interface because the packets are forwarded on VCs in different subinterfaces.

Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode: Point-to-point - A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router. Point-to-point - A single point-to-point subinterface establishes one PVC connection to another physical interface or subinterface on a remote router. In this case, each pair of the point-to-point routers is on its own subnet, and each point-to-point subinterface has a single DLCI. In this case, each pair of the point-to-point routers is on its own subnet, and each point-to-point subinterface has a single DLCI. In a point-to-point environment, each subinterface is acting like a point-to-point interface. Typically, there is a separate subnet for each point-to-point VC. In a point-to-point environment, each subinterface is acting like a point-to-point interface. Typically, there is a separate subnet for each point-to-point VC. Therefore, routing update traffic is not subject to the split horizon rule. Therefore, routing update traffic is not subject to the split horizon rule.

Frame Relay subinterfaces can be configured in either point-to-point or multipoint mode: Multipoint - A single multipoint subinterface establishes multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. Multipoint - A single multipoint subinterface establishes multiple PVC connections to multiple physical interfaces or subinterfaces on remote routers. All the participating interfaces are in the same subnet. All the participating interfaces are in the same subnet. The subinterface acts like an NBMA Frame Relay interface, so routing update traffic is subject to the split horizon rule. The subinterface acts like an NBMA Frame Relay interface, so routing update traffic is subject to the split horizon rule. Typically, all multipoint VCs belong to the same subnet. Typically, all multipoint VCs belong to the same subnet.

Access rate or port speed & Committed Information Rate (CIR) From a customer's point of view then, Frame Relay is an interface and one or more PVCs. Customers simply buy Frame Relay services from a service provider. From a customer's point of view then, Frame Relay is an interface and one or more PVCs. Customers simply buy Frame Relay services from a service provider. Access rate or port speed - From a customer's point of view, the service provider provides a serial connection or access link to the Frame Relay network over a leased line. Access rate or port speed - From a customer's point of view, the service provider provides a serial connection or access link to the Frame Relay network over a leased line. The speed of the line is the access speed or port speed. The speed of the line is the access speed or port speed. Access rate is the rate at which your access circuits join the Frame Relay network. Access rate is the rate at which your access circuits join the Frame Relay network. These are typically at 56 kb/s, T1 (1.536 Mb/s), or Fractional T1 (a multiple of 56 kb/s or 64 kb/s). These are typically at 56 kb/s, T1 (1.536 Mb/s), or Fractional T1 (a multiple of 56 kb/s or 64 kb/s). Port speeds are clocked on the Frame Relay switch. It is not possible to send data at higher than port speed. Port speeds are clocked on the Frame Relay switch. It is not possible to send data at higher than port speed. Committed Information Rate (CIR) - Customers negotiate CIRs with service providers for each PVC. Committed Information Rate (CIR) - Customers negotiate CIRs with service providers for each PVC. The CIR is the amount of data that the network receives from the access circuit. The CIR is the amount of data that the network receives from the access circuit. The service provider guarantees that the customer can send data at the CIR. All frames received at or below the CIR are accepted. The service provider guarantees that the customer can send data at the CIR. All frames received at or below the CIR are accepted.

Bursting A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge. A great advantage of Frame Relay is that any network capacity that is being unused is made available or shared with all customers, usually at no extra charge. Because the physical circuits of the Frame Relay network are shared between subscribers, there will often be time where there is excess bandwidth available. Because the physical circuits of the Frame Relay network are shared between subscribers, there will often be time where there is excess bandwidth available. Frame Relay can allow customers to dynamically access this extra bandwidth and "burst" over their CIR for free. Frame Relay can allow customers to dynamically access this extra bandwidth and "burst" over their CIR for free. Bursting allows devices that temporarily need additional bandwidth to borrow it at no extra cost from other devices not using it. For example, if PVC 102 is transferring a large file, it could use any of the 16 kb/s not being used by PVC 103. Bursting allows devices that temporarily need additional bandwidth to borrow it at no extra cost from other devices not using it. For example, if PVC 102 is transferring a large file, it could use any of the 16 kb/s not being used by PVC 103. A device can burst up to the access rate and still expect the data to get through. The duration of a burst transmission should be short, less than three or four seconds. A device can burst up to the access rate and still expect the data to get through. The duration of a burst transmission should be short, less than three or four seconds.

Bursting - Committed Burst Information Rate (CBIR) The CBIR is a negotiated rate above the CIR which the customer can use to transmit for short burst. The CBIR is a negotiated rate above the CIR which the customer can use to transmit for short burst. It allows traffic to burst to higher speeds, as available network bandwidth permits. It allows traffic to burst to higher speeds, as available network bandwidth permits. However, it cannot exceed the port speed of the link. However, it cannot exceed the port speed of the link. A device can burst up to the CBIR and still expect the data to get through. A device can burst up to the CBIR and still expect the data to get through. The duration of a burst transmission should be short, less than three or four seconds. If long bursts persist, then a higher CIR should be purchased. The duration of a burst transmission should be short, less than three or four seconds. If long bursts persist, then a higher CIR should be purchased.

Bursting - Excess Burst Size (BE) The BE is the term used to describe the bandwidth available above the CBIR up to the access rate of the link. The BE is the term used to describe the bandwidth available above the CBIR up to the access rate of the link. Unlike the CBIR, it is not negotiated. Unlike the CBIR, it is not negotiated. Frames may be transmitted at this level but will most likely be dropped. Frames may be transmitted at this level but will most likely be dropped.

Frame Relay Flow Control Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-VC flow control. Frame Relay reduces network overhead by implementing simple congestion-notification mechanisms rather than explicit, per-VC flow control. These congestion-notification mechanisms are the Forward Explicit Congestion Notification (FECN) and the Backward Explicit Congestion Notification (BECN). These congestion-notification mechanisms are the Forward Explicit Congestion Notification (FECN) and the Backward Explicit Congestion Notification (BECN). FECN and BECN are each controlled by a single bit contained in the frame header. FECN and BECN are each controlled by a single bit contained in the frame header. They let the router know that there is congestion and that the router should stop transmission until the condition is reversed. They let the router know that there is congestion and that the router should stop transmission until the condition is reversed. BECN is a direct notification. FECN is an indirect one. BECN is a direct notification. FECN is an indirect one.

Frame Relay Flow Control cont. The frame header also contains a Discard Eligibility (DE) bit, which identifies less important traffic that can be dropped during periods of congestion. The frame header also contains a Discard Eligibility (DE) bit, which identifies less important traffic that can be dropped during periods of congestion. DTE devices can set the value of the DE bit to 1 to indicate that the frame has lower importance than other frames. DTE devices can set the value of the DE bit to 1 to indicate that the frame has lower importance than other frames. When the network becomes congested, DCE devices discard the frames with the DE bit set to 1 before discarding those that do not. When the network becomes congested, DCE devices discard the frames with the DE bit set to 1 before discarding those that do not. This reduces the likelihood of critical data being dropped during periods of congestion. This reduces the likelihood of critical data being dropped during periods of congestion. In periods of congestion, the provider's Frame Relay switch applies the following logic rules to each incoming frame based on whether the CIR is exceeded: In periods of congestion, the provider's Frame Relay switch applies the following logic rules to each incoming frame based on whether the CIR is exceeded: If the incoming frame does not exceed the CIBR, the frame is passed. If the incoming frame does not exceed the CIBR, the frame is passed. If an incoming frame exceeds the CIBR, it is marked DE. If an incoming frame exceeds the CIBR, it is marked DE. If an incoming frame exceeds the CIBR plus the BE, it is discarded. If an incoming frame exceeds the CIBR plus the BE, it is discarded.

Frame Relay Flow Control cont. Frames arriving at a switch are queued or buffered prior to forwarding. As in any queuing system, it is possible that there will be an excessive buildup of frames at a switch. This causes delays. Delays lead to unnecessary retransmissions that occur when higher level protocols receive no acknowledgment within a set time. In severe cases, this can cause a serious drop in network throughput. To avoid this problem, Frame Relay incorporates a flow control feature. Frames arriving at a switch are queued or buffered prior to forwarding. As in any queuing system, it is possible that there will be an excessive buildup of frames at a switch. This causes delays. Delays lead to unnecessary retransmissions that occur when higher level protocols receive no acknowledgment within a set time. In severe cases, this can cause a serious drop in network throughput. To avoid this problem, Frame Relay incorporates a flow control feature. To reduce the flow of frames to the queue, the switch notifies DTEs of the problem using the Explicit Congestion Notification bits in the frame address field. To reduce the flow of frames to the queue, the switch notifies DTEs of the problem using the Explicit Congestion Notification bits in the frame address field. The FECN bit is set on every frame that the switch receives on the congested link. The FECN bit is set on every frame that the switch receives on the congested link. The BECN bit is set on every frame that the switch places onto the congested link. The BECN bit is set on every frame that the switch places onto the congested link. DTEs receiving frames with the ECN bits set are expected to try to reduce the flow of frames until the congestion clears. DTEs receiving frames with the ECN bits set are expected to try to reduce the flow of frames until the congestion clears. If the congestion occurs on an internal trunk, DTEs may receive notification even though they are not the cause of the congestion. If the congestion occurs on an internal trunk, DTEs may receive notification even though they are not the cause of the congestion.